Amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease, is an adult-onset neurodegenerative disease that affects upper and lower motor neurons. The key clinical features in ALS patients include muscle wasting, and progressive loss of spinal motor neurons and upper motor neurons and their axons in the lateral columns of the spinal cord. The past 10 years have witnessed a tremendous expansion in the molecular mechanisms of this devastating disease thanks to the discoveries of genetic mutations that are causally linked to both familial ALS (FALS) and sporadic ALS (SALS). Characterizations of these ALS disease genes suggest that dysfunctions in protein homeostasis via the ubiquitin-proteasome pathways (proteostasis) might contribute to the pathogenesis and disease progression in ALS. Consistent with the genetic data, a key pathological feature in FALS and SALS is accumulation of misfolded proteins in motor neurons, which disrupts normal neuronal functions, including axonal transport, mitochondrial bioenergetics, gene expression, and synaptic connectivity. Persistent accumulation of misfolded proteins eventually triggers endoplasmic reticulum (ER) stress-induced cell death, which leads to neurodegeneration through mechanisms that are poorly understood. To investigate how misfolded proteins promote neurodegeneration, we have focused on the IRE1?-ASK1-JNK pathway because of its role in ER stress-induced cell death. Our results showed that a highly conserved kinase HIPK2 (homeodomain interacting protein kinase 2) acts downstream of IRE1?-ASK1 and upstream of JNK to promote ER stress-mediated cell death. Using proteomics and phospho-peptide mapping, we showed that ER stress activates HIPK2 by promoting phosphorylation on specific Serine and Threonine residues within the kinase domain of HIPK2. Mutagenesis analyses confirmed that phosphorylation of these amino acids in HIPK2 is prerequisite to promote ER stress-mediated neuronal cell death. To investigate the role of HIPK2 in ALS, we have developed phospho-HIPK2-specific antibodies and showed that HIPK2 activation can be detected in the spinal cord of SOD1G93A mice before symptom onset. Furthermore, the level of phospho-HIPK2 positively correlates with disease progression in SOD1G93A mice, suggesting that HIPK2 activation may directly promote neurodegeneration. In support of this idea, loss of HIPK2 significantly protects cultured spinal motor neurons from SOD1G93A-induced cell death. Given the implications of ER stress in both FALS and SALS, our results support the hypotheses that (1) HIPK2 is an essential target in the downstream of IRE1? pathway that promotes ER stress-induced neurodegeneration, and (2) HIPK2 activation may have broader implications in the pathogenesis of, and in the therapeutics for, both FALS and SALS.